A modularly structured ISDN communication system has a system processor that is administered by a multi-tasking real-time operating system. Since this operating system can only assign system-internal function programs to the system processor (on-line system), a software application module having a time-sharing operating system is provided, so that the same processor can be administered by both operating systems. In order to effect this, one task of the real-time operating system is fashioned as a sluice sub-system SUB in which an allocation switch from the addressing tables of the real-time operating system onto those of the time-sharing operating system occurs. An interrupt transition routine that leads back into the sluice sub-system SUB in which the allocation switch is in turn cancelled is present in the time-sharing operating system for a processing of interrupt requests. Messages can be forwarded from one operating system to the other by use of an intercommunication data segment, so that data from the administration of the one operating system can be transferred into that of the other operating system in a simple manner.